Cholesterol side-chain cleavage enzyme is commonly referred to as P450scc, where "scc" is an acronym for side-chaincleavage. P450scc is a mitochondrialenzyme that catalyzes conversion of cholesterol to pregnenolone. This is the first reaction in the process of steroidogenesis in all mammalian tissues that specialize in the production of various steroid hormones.
The highest level of the cholesterol side-chain cleavage system is found in the adrenal cortex and the corpus luteum. The system is also expressed at high levels in steroidogenic theca cells in the ovary, and Leydig cells in the testis. During pregnancy, the placenta also expresses significant levels of this enzyme system. P450scc is also present at much lower levels in several other tissue types, including the brain. In the adrenal cortex, the concentration of adrenodoxin is similar to that of P450scc, but adrenodoxin reductase is expressed at lower levels.
Immunofluorescence studies using specific antibodies against P450scc system enzymes have demonstrated that proteins are located exclusively within the mitochondria. P450scc is associated with the inner mitochondrial membrane, facing the interior (matrix). Adrenodoxin and adrenodoxin reductase are soluble peripheral membrane proteins located inside the mitochondrial matrix that appear to associate with each other primarily through electrostatic interactions.
Mechanism of action
P450scc catalyzes the conversion of cholesterol to pregnenolone in three monooxygenase reactions. These involve 2 hydroxylations of the cholesterol side-chain, which generate, first, 22R-hydroxycholesterol and then 20alpha,22R-dihydroxycholesterol. The final step cleaves the bond between carbons 20 and 22, resulting in the production of pregnenolone and isocaproic aldehyde.
The involvement of three proteins in cholesterol side-chain cleavage reaction raises the question of whether the
three proteins function as a ternary complex as reductase:adrenodoxin:P450. Both spectroscopic studies of adrenodoxin binding to P450scc and kinetic studies in the presence of varying concentrations of adrenodoxin reductase demonstrated that the reductase competes with P450scc for binding to adrenodoxin. These results demonstrated that the formation of a functional ternary complex is not possible. From these studies, it was concluded that the binding sites of adrenodoxin to its reductase and to P450 are overlapping and, as a consequence, adrenodoxin functions as a mobile electron shuttle between reductase and P450. These conclusions have been confirmed by structural analysis of adrenodoxin and P450 complex.
The process of electron transfer from NADPH to P450scc is not tightly coupled; that is, during electron transfer from adrenodoxin reductase via adrenodoxin to P450scc, a certain portion of the electrons leak outside of the chain and react with O2, generating superoxide radicals. Steroidogenic cells include a diverse array of antioxidant systems to cope with the radicals generated by the steroidogenic enzymes.
P450scc is always active, however its activity is limited by the supply of cholesterol in the inner membrane. The supplying of cholesterol to this membrane (from the outer mitochondrial membrane) is, thus, considered the true rate-limiting step in steroid production. This step is mediated primarily by the steroidogenic acute regulatory protein (StAR or STARD1). Upon stimulation of a cell to make steroid, the amount of StAR available to transfer cholesterol to the inner membrane limits how fast the reaction can go (the acute phase). With prolonged (chronic) stimulation, it is thought that cholesterol supply becomes no longer an issue and that the capacity of the system to make steroid (i.e., level of P450scc in the mitochondria) is now more important.
Corticotropin (ACTH) is a hormone that is released from the anterior pituitary in response to stress situations. A study of the steroidogenic capacity of the adrenal cortex in infants with acute respiratory disease demonstrated that indeed during disease state there is a specific increase in the steroidogenic capacity for the synthesis of the glucocorticoid cortisol but not for the mineralocorticoid aldosterone or androgen DHEAS that are secreted from other zones of the adrenal cortex.
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